This application claims foreign priority benefits under Title 35, United States Code xc2xa7119, of German Application 198 55 140.1 filed on Nov. 30, 1998, and of PCT Application No. PCT/DE99/03774 filed on Nov. 29, 1999.
1. Technical Field of the Invention
The invention concerns an apparatus for scanning an object with a scanning beam which is guided into a rotating diverter unit and deflected with a movably mounted deflector unit at a controllable angle of deflection onto the object.
2. Description of the Related Art
Corresponding apparatuses based on the optical principle are known in particular as laser scanners, and are used e.g. for laser-radar applications. Here an object is scanned with a coherent light source, i.e. a laser beam, and the distance from the scanned measuring points is determined by a laser-optics rangefinder. Thus it is possible to produce a three-dimensional image of the scanned surface of the object of measurement.
Corresponding diverter units are also used for laser printers or for writing on, checking and playing compact discs.
In EP 575 988 A1 is disclosed a laser scanner for laser printers and in EP 178 037 A2 is disclosed a corresponding laser scanner for the quality control of semiconductor wafers, in which a laser beam is diverted with a rotating polygon mirror onto a stationary elongate deflector mirror and guided onto the printer drum. On rotation of the polygon mirror, the laser beam sweeps across the longitudinal axis of the deflector mirror and one-dimensional scanning takes place.
So-called galvanometer scanners in which the above-mentioned deflector mirror is tiltable are known for scanning a two-dimensional surface. This traditional embodiment is shown in FIG. 4. By rapid rotation of the polygon mirror and slow rotation or tilting of the deflector mirror, the object is two-dimensionally scanned in each case line by line in Cartesian coordinates.
In the above-mentioned laser scanners, the deflector mirror must be relatively wide due to diversion of the laser beam by the polygon mirror. Since the drive motors for both mirrors must be integrated in the laser scanner, the designs are quite large. Also coordinated control of the mirrors is elaborate.
In EP 219 620 A2 is described a rapid scanning device for maximum diversion speeds, in which a laser beam is diverted with two symmetrical cylinder mirrors which are set in rotation. Here the mirror coatings are at a small angle to the axis of rotation, so that the airborne cylinder mirror which is set in rotation by an air stream wobbles and diverts the laser beam onto a circular path. The laser beam is guided onto a corresponding second cylinder mirror, so that a three-dimensional pattern is formed as a function of the respective speeds of revolution and mirror diameters.
In WO 96/11420 is described a laser scanner for two-dimensional room scanning, in which a laser beam is diverted with two mirrors standing relative to each other. The diverter units are controlled with a computer system. This requires high expenditure on control. Also the space requirements for the two diverter units and the manufacturing costs are relatively high.
For scanning bar codes, in WO 89/05512 is described a rotary scanner in which a laser beam is guided into an inclined mirror which is arranged in a rotating cylindrical housing. The laser beam emerges from an opening on the circumference of the housing and encounters a number of inclined deflector mirrors which form a pyramid with a polygonal base surface. The rigidly mounted deflector mirrors deflect the laser beam onto the object, wherein the inclination and the number of deflector mirrors determine the scan pattern. The scan surface is however here disadvantageously also limited by a narrow region for e.g. a bar code, and it is not possible to scan a complete surface systematically.
In DE-A-37 05 042 A1 is described an optical scanning apparatus in which diverter mirrors are arranged on a rotatable carrier. The diverter mirrors are in each case arranged at the same non-right-angled angle to a radius of the carrier, so that a scanning beam is diverted onto a tilt mirror at an angle altered by the rotation. The tilt mirror is movably attached to a stepping motor. The tilt angle of the tilt mirror is discretely varied as soon as a horizontal path has been scanned by rotation of the carrier. The angle of the diverter mirrors, the speed of rotation of the carrier and the tilt angle of the tilt mirror must be synchronized with each other very precisely in order to be able to scan a surface in a controlled manner. Also the size of the scanning apparatus is relatively large, particularly because the tilt mirror is actively operated with the stepping motor.
In U.S. Pat. No. 5,071,239 is described a scanning telescope in which a scanning beam is guided onto a diverter mirror and directed by a deflector unit onto a lens. The deflector unit has a curved surface, so that the deflected laser beam is diverted from the normal. The deflector unit is mounted stationarily.
It is therefore possible to scan a circular path with the arrangement. The scanning speed here depends on the speed of rotation of the diverter mirror. The radius of the scanned circular path can be controlled by horizontal displacement of the diverter mirror 11. This disadvantageously requires elaborate coordinated control and an additional actuator.
It was therefore the aspect of the present invention to provide an apparatus for scanning a surface with a scanning beam, which allows a two-dimensional scan path with only one drive unit. The apparatus should have a small size, require little expenditure on control and be able to detect and scan the surface of an object completely.
The aspect is achieved by the apparatus with the characteristics of patent claim 1. Advantageous embodiments are described in the subsidiary claims and the specification. The above listed aspect is just one of the innumerable aspects associated with the present invention and should not be considered as all-inclusive.
The deflector unit is arranged in such a way that the angle of deflection of the deflector unit is adjusted as a function of the centrifugal force and so as a function of the speed of the diverter unit. Hence the scan path is fixed by the speed of rotation of the drive for the diverter unit, and advantageously an additional drive for the deflector unit or additional control means can be eliminated. The apparatus can thus be integrated in small assemblies. Also the expenditure on control and components is minimal.
The diverter unit is advantageously arranged in a housing rotatable about an axis of rotation of the housing. The deflector unit is attached to the circumference of the housing, so that speed-related centrifugal forces act on the deflector unit and bend it. The diverter unit and the deflector unit are in this case aligned with each other in such a way that the scanning beam is guided from the diverter unit onto the deflector unit and deflected by the latter at a speed-related angle onto the object.
The deflector unit is for this purpose advantageously attached to one end of a spring arm. The other end of the spring arm is attached to the housing. The material properties of the spring arm then determine the degree of diversion in proportion to the speed-related centrifugal force acting on the deflector unit.
The housing preferably has a recess for receiving the spring arm, so that the spring arm is integrated in the housing and the housing surface ends flush.
The spring arm can for example be made of plastic or metal. Advantageously the spring arm is a leaf spring. The bending properties of the leaf spring determine the speed-related diversion behaviour.
To compensate for the imbalance of the housing, which is caused amongst other things by a spring arm, at least one counterweight is provided on the circumference of the housing.
Preferably two additional spring arms which are mounted on the circumference of the housing at an angle of 120xc2x0 each in relation to the spring arm, are provided as the counterweights. At the free ends of the additional spring arms are respectively provided two weights.
The housing is advantageously a hollow cylinder through which the scanning beam can be guided. When using an optical measuring beam, e.g. a laser beam, it is advantageous if the diverter unit is a prism arranged in the hollow cylinder and the deflector unit is a mirror.